Power dissipating elements such as resistors and varistors are of interest to use in various power transmission situations in order to dissipate generated power when there is a fault along the power transmission path.
One exemplifying situation when such an element or circuit may be of interest to use is in the case when there is an isolated power generation system, such as a windfarm or a hydro power solar panel farm connected to an alternating current (AC) transmission system via a direct current (DC) transmission system, such as a High Voltage Direct Current (HVDC) power transmission system.
In this case, if there is a fault in the AC or in the DC system, it is possible to add a resistive circuit to the AC system or a chopper circuit to the DC system. Both the resistive circuit and the chopper circuit would then comprise a resistor and a switch for connecting the resistor to the system in question. In both cases the resistor dissipates energy generated in the power generation system and supports the frequency stability of the AC system.
In power transmission from remote power generation centers where DC overhead lines are used, DC faults will be critical related to the complete interruption of power transmission resulting in over-frequency conditions of the generating units. A traditional DC chopper connected on a DC bus of the DC transmission system will not be able to absorb active power due to the low DC voltage.
Traditionally such choppers have thus been placed in the DC system. An example of this can be seen in CN 204179684.
For HVDC the cost of the switch becomes substantial due to the high voltage capability required and that the switch requires direct current breaking capability. Solutions to achieve integration of the chopper functionality in the main converters would therefore be desirable.
One attractive placing of a chopper circuit is thus in a converter, such as a voltage source converter converting between AC and DC and being placed in such a DC transmission system as a part of an interface to the AC power generating system. This type of converter is often a modular multilevel converter.
The article “HVDC and Power Electronic Systems for Overhead Line and Insulated Cable Applications” by J. Dorn et al, CIGRE Colloquium on H VDC and Power Electronic Systems, San Francisco 2012 suggests the use of a modularized chopper in such a voltage source converter. However, the modularization of choppers is in many situations not economical due to the additional power electronic switches required or feasible related to the thermal stresses and cooling requirements.
There is in view of this a need for an alternative chopper realization in a converter converting between AC and DC and provided as a part of an interface to a power generating system.